Methods for fabricating monolithic device containing circuitry and suspended microstructure

ABSTRACT

A monolithic capacitance-type microstructure includes a semiconductor substrate, a plurality of posts extending from the surface of the substrate, a bridge suspended from the posts, and an electrically-conductive, substantially stationary element anchored to the substrate. The bridge includes an element that is laterally movable with respect to the surface of the substrate. The substantially stationary element is positioned relative to the laterally movable element such that the laterally movable element and the substantially stationary element form a capacitor. Circuitry may be disposed on the substrate and operationally coupled to the movable element and the substantially stationary element for processing a signal based on a relative positioning of the movable element and the substantially stationary element. A method for fabricating the microstructure and the circuitry is disclosed.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of pending application Ser. No.08/448,181, Filed May 23, 1995, which is a continuation of applicationSer. No. 08/075,043 filed Jun. 10, 1993, U.S. Pat. No. 5,417,111, whichis a division of application Ser. No. 07/899,765 filed Jun. 17, 1992,abandoned, which is a continuation-in-part of application Ser. No.07/569,080 filed Aug. 17, 1990, abandoned, and a continuation-in-part ofapplication Ser. No. 07/872,037 filed Apr. 22, 1992, U.S. Pat. No.5,314,572, which is a continuation-in-part of application Ser. No.07/569,080 filed Aug. 17, 1990 abandoned.

FIELD OF THE INVENTION

This invention relates to monolithic micromechanical devices and, moreparticularly, to methods for fabricating monolithic micromechanicaldevices which include suspended microstructures and which may includecircuitry.

BACKGROUND OF THE INVENTION

The construction of microsensors on microchips is of great interest inmany industries because of its potential to reduce size and cost ofdevices which require the sensing of environmental or other conditions.An accelerometer is just one example of a type of sensor which has wideapplication possibilities.

Acceleration sensors are known for measuring force or mass or to operatecontrol systems responsive to acceleration. For instance, accelerationsensors may be used in automotive vehicles to deploy air bags responsiveto a particular threshold deceleration rate of a vehicle. Accelerationsensors may also be used in the automotive industry as part of activesuspension systems in which microcontrollers adjust suspensioncomponents responsive to the vertical acceleration of the wheels.

An accelerometer comprises an acceleration-sensing element, ortransducer, which is commonly interfaced to resolving circuitry forproducing a useful output signal from the output of the transducer.Though the term "accelerometer" is sometimes used to refer to the sensor(or transducer) itself, the term is used herein to denote a completesystem including a transducer as well as the resolving circuitry.

Many commercially available accelerometers employ transducers comprisinga mechanical or electromechanical element (e.g., piezoelectric,piezoresistive or strain gauge).

Acceleration sensing microstructures embodied on silicon chips have beensuggested in the prior art. For instance, U.S. Pat. No. 4,711,128 issuedto Boura discloses an acceleration sensor formed by micromachining afine monocrystal wafer. The sensor comprises a flat mobile masssuspended above the rest of the structure by means of two thin parallelstrips situated on each side of the mass. The mass comprises at leastone mobile capacitive plate which is disposed between two othercapacitive plates which are not part of the suspended microstructure butare fixed on the structure. The mobile plates are charged to a voltagerelative to the stationary plates. When the sensor is subjected to anacceleration, the mobile plates move relative to the fixed platescausing a change in capacitance between the mobile plates and each ofthe fixed plates. The change in capacitance is observed by observing thevoltage between the mobile plate and the fixed plates and is a directindication of the distance of movement of the fixed plate which, inturn, is a measurement of the acceleration.

U.S. Pat. No. 4,705,659 issued to Bernstein et al. teaches a techniqueof fabricating a free standing thin or thick structure such as anacceleration sensor, including the steps of providing a substrate,forming a layer of carbon on the substrate and depositing a film ofpolycrystalline material over the layer of carbon. The sandwichstructure is heated in an oxidizing ambient to cause the oxidation ofthe carbon layer, leaving the polysilicon material as a free-standingfilm.

The prior art, however, does not teach a monolithic accelerometer inwhich the acceleration sensor as well as the resolving circuitry forproducing a useful output are embodied on a single chip or a techniquefor making such a monolithic accelerometer. Thus, prior artaccelerometers require a separate chip or other means containingcircuitry for resolving the output of the sensor into a usable signal.

SUMMARY OF THE INVENTION

According to a first aspect of the invention, a process for fabricatinga monolithic device is provided. The process comprises the steps ofproviding a substrate having a first surface region for formation of acircuit and a second surface region for formation of a suspendedmicrostructure, forming transistors in the first surface region of thesubstrate, forming a spacer layer over the second surface region of thesubstrate, forming at least one anchor opening in the spacer layer,forming a patterned polysilicon layer over the spacer layer and theanchor opening to define the microstructure, forming conductive pathsfor electrically interconnecting the transistors to form the circuit andfor electrically interconnecting the microstructure to the circuit, andremoving the spacer layer to release the suspended microstructure.

According to another aspect of the invention, a process for fabricatinga suspended microstructure on a substrate is provided. The processcomprises the steps of forming a spacer layer over the substrate,forming at least one anchor opening in the spacer layer, forming apolysilicon layer over the spacer layer and the anchor opening,implanting ions into the polysilicon layer to establish a desiredconductivity of the polysilicon layer, patterning the polysilicon layerto define the suspended microstructure, and removing the spacer layer torelease the suspended microstructure.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a top view of the suspended portion of an exemplarymicrostructure which can be fabricated by the process of the presentinvention.

FIG. 1B is a side view of the exemplary suspended microstructure portionof FIG. 1A which can be fabricated by the method of the presentinvention.

FIG. 2A is a high-level block diagram of a first embodiment of adifferential-capacitor accelerometer according to the present invention.

FIG. 2B is a simplified diagrammatic top view of an exemplarydifferential-capacitor accelerometer according to the invention.

FIG. 2C is a simplified diagrammatic top view of a portion of a sensoraccording to FIG. 2B, with multiple capacitance cells.

FIG. 2D is a partially block, partially schematic circuit diagram of afirst exemplary embodiment of the accelerometer of FIG. 2A.

FIG. 2E is a partially diagrammatic, partially schematic circuit modelof a second embodiment of an accelerometer according to the presentinvention.

FIG. 2F is a partially block, partially schematic circuit diagram of theembodiment of FIG. 2E.

FIG. 3 is a cross-sectional view of the circuit region of an exemplarychip during a first stage of the fabrication method of the presentinvention.

FIG. 4 is a cross-sectional view of the sensor region of the sensorregion of an exemplary chip during a second stage of the fabricationmethod of the present invention.

FIG. 5 is a cross-sectional view of the circuit region of an exemplarychip during a third stage of the fabrication method of the presentinvention.

FIG. 6 is a cross-sectional view of the sensor region of the sensorregion of an exemplary chip during a fourth stage of the fabricationmethod of the present invention.

FIG. 7 is a cross-sectional view of the sensor region of the sensorregion of an exemplary chip during a fifth stage of the fabricationmethod of the present invention.

FIG. 8 is a cross-sectional view of the sensor region of the sensorregion of an exemplary chip during a sixth stage of the fabricationmethod of the present invention.

FIG. 9 is a cross-sectional view of the sensor region of the sensorregion of an exemplary chip during a seventh stage of the fabricationmethod of the present invention.

FIG. 10 is a cross-sectional view of the sensor region of the sensorregion of an exemplary chip during an eighth stage of the fabricationmethod of the present invention.

FIG. 11 is a cross-sectional view of the sensor region of the sensorregion of an exemplary chip during a ninth stage of the fabricationmethod of the present invention.

FIG. 12 is a cross-sectional view of the sensor region of the sensorregion of an exemplary chip during a tenth stage of the fabricationmethod of the present invention.

FIG. 13 is a cross-sectional view of the sensor region of the sensorregion of an exemplary chip during an eleventh stage of the fabricationmethod of the present invention.

FIG. 14 is a cross-sectional view of the sensor region of the sensorregion of an exemplary chip during a twelfth stage of the fabricationmethod of the present invention.

FIG. 15 is a cross-sectional view of the sensor region of the sensorregion of an exemplary chip during a thirteenth stage of the fabricationmethod of the present invention.

FIG. 16 is a cross-sectional view of the sensor region of the sensorregion of an exemplary chip during a fourteenth stage of the fabricationmethod of the present invention.

FIG. 17 is a cross-sectional view of the sensor region of the sensorregion of an exemplary chip during a fifteenth stage of the fabricationmethod of the present invention.

FIG. 18 is a cross-sectional view of the circuit region of an exemplarychip during a sixteenth stage of the fabrication method of the presentinvention.

FIG. 19 is a cross-sectional view of the sensor region of the sensorregion of an exemplary chip during a seventeenth stage of thefabrication method of the present invention.

FIG. 20 is a cross-sectional view of the sensor region of the sensorregion of an exemplary chip during an eighteenth stage of thefabrication method of the present invention.

FIG. 21 is a cross-sectional view of the sensor region of the sensorregion of an exemplary chip during a nineteenth stage of the fabricationmethod of the present invention.

FIG. 22 is a cross-sectional view of the sensor region of the sensorregion of an exemplary chip during a twentieth stage of the fabricationmethod of the present invention.

FIG. 23 is a cross-sectional view of the sensor region of the sensorregion of an exemplary chip during a twenty-first stage of thefabrication method of the present invention.

DETAILED DESCRIPTION

U.S. patent application Ser. No. 07/569,080 filed Aug. 17, 1990 andentitled MONOLITHIC ACCELEROMETER is assigned to the same assignee asthis application, and the disclosure of that application is incorporatedherein by reference. That patent application discloses a monolithicaccelerometer which can be fabricated by the method disclosed herein.Further, U.S. patent application Ser. No. 07/872,037 entitled METHOD FORFABRICATING MICROSTRUCTURES, filed on Apr. 22, 1992 and assigned to thesame assignee as the present application, discloses an improvement onthe method disclosed herein relating to a method for preventing thesuspended microstructure from becoming damaged or stuck to the substrateduring the fabrication processes. The disclosure of that patentapplication is also incorporated herein by reference.

FIGS. 1A and 1B are top and side views, respectively, of the suspendedportion 10 of an exemplary microstructure which may be fabricated by themethod of the present invention. As shown in the drawings, the suspendedcentral beam 12 has a plurality of suspended arms 14 extendingtransversely therefrom. The beam 12 is supported at opposite ends fromsupport beams 16 and 18. Support beams 16 and 18 are suspended above thesilicon substrate by anchors 20, 22, 24 and 26.

FIGS. 1A and 1B show only the suspended portion of the microstructure.However, in an actual commercially usable device, the microstructurewould also comprise fixed components. Referring now to FIG. 2A, ahigh-level block diagram is shown of a differential-capacitoraccelerometer 10 according to the present invention. The accelerometer10 comprises a signal source 1, sensor 2 having first and seconddifferential capacitors 200 and 300, and a signal resolver 3.Differential capacitors 200 and 300 are fabricated so that one electrodeof each moves when force is applied, such that one capacitance increasesand the other decreases. The signal source 1 drives the capacitors 200,300 with sinusoidal signals of equal frequency, phase and amplitude, butof opposite polarities. Consequently, the amplitude and phase of thesignal at the junction 4 of the differential capacitors is a function ofthe difference in capacitances, which is directly related to theforce-induced displacement of the capacitor electrodes due toacceleration. The signal resolver processes this signal, to generatetherefrom a signal proportional to the acceleration of the capacitorplates relative to the substrate and package of the assembly.

Each of the electrodes of a differential capacitor sensor according tothe present invention is formed of a plurality of segments which arearranged in such a manner that each capacitor is built up from aplurality of smaller capacitance "cells" connected in parallel. FIG. 2Bshows a top view of an exemplary differential-capacitor sensor 5according to the invention, but with only a single capacitance cellbeing depicted, to avoid unnecessary obfuscation of the inventiveconcept. On top of a silicon substrate 8, a suspended polysilicon "beam"6 is formed. (The method of forming this suspended structure isdiscussed below.) Beam 6 rests above the surface of the substrate, onfour posts, or anchors, 7A-7D, indicated by the "X" symbols in thefigure. Beam 6 is generally H-shaped, with two elongated, narrow legs 16and 18, and a transverse central member 12 suspended between them.Central member 12 is typically much stiffer and more massive than legs16 and 18. A pair of beam fingers 14A and 14B depend in parallelorientation from central member 12, transversely to the axis 93 of thebeam. Finger 14A forms one electrode of a parallel plate capacitor,having a stationary member 29A as its opposite electrode, or plate.Similarly, finger 14B forms one electrode of a second parallel platecapacitor, having a stationary member 28B as its opposite plate. Notethat fingers 14A and 14B are connected together both physically andelectrically and are, thus, a common electrode.

Electrical connection is made to fingers 29A and 28B via a heavilyn+doped region 9A and the polysilicon bridge itself. Electricalconnection is made to plate 29A via a heavily n+doped region 9B, andconnection to plate 28A is made via a similar region 9C. As will beshown below, regions 9B and 9C may be extended to connect together, inparallel, the similar members of other capacitance cells.

An n+doped region 91 also is provided beneath the entire polysiliconbridge structure, including the capacitance cells, as a bootstrapdiffusion for reducing parasitic capacitance from the beam to thesubstrate. This is necessitated, at least in part, by the very lowvalues of the capacitance per cell. In the example of FIGS. 2B and 2C,using the dimensions provided herein, each capacitor has a nominalcapacitance of about 0.002 pF. With 56 cells in parallel, the totalcapacitance at rest is only about 0.1 pF. A full scale measurementinvolves only about an eight percent change in the value of eachcapacitor when the sensor is operated open-loop; naturally, inclosed-loop operation the change is about ten times less.

The extension of the sensor architecture of FIG. 2B to multiple cells isillustrated in FIG. 2C for a four-cell example, the four cells beinglabeled 62A-62D.

The approximate dimensions below may be used to fabricate such a sensor,the dimension labels pertaining to the features shown on FIG. 2B:

d₁ =300 micrometers

d₂ =2.0 micrometers

d₃ =40 micrometers

d₄ =450 micrometers

d₅ =125 micrometers

d₆ =8 micrometers

d₇ =5 micrometers

d₈ =3 micrometers

For purposes of reference in the discussion below, the heavily-doped, ormetallization, regions 9A, 9B, 9C, and 91 are shown as terminating atterminals 72, 74, 76, and 80, respectively, though it should beunderstood that no actual connection terminal need be present at thosephysical locations.

When a force is applied to the substrate 8, in the x-direction, thesubstrate and plates move in that direction while the beam 12 tends toremain in its prior condition. Motion of the beam relative to thesubstrate is permitted by the fact that legs 16 and 18 are notabsolutely rigid and will deflect slightly. When the force is in thepositive x-direction, the separation between finger 14A and plate 29Aincreases, decreasing the capacitance in the capacitor they form;conversely, the separation between finger 14B and plate 28B decreases,increasing the capacitance in the capacitor they form.

FIG. 2D provides a partially block-, partially schematic circuit diagramshowing, in greater detail, a first (open-loop) embodiment of signalconditioning circuitry for use with the sensor of FIGS. 2B and 2C, foran accelerometer. The oscillator 100 supplies an approximately 1 MHZsinusoidal signal to a carrier generator 102. The carrier generatorsupplies therefrom two 1 MHZ sinusoidal output signals 180 degrees outof phase from each other; thus, the output signals are of the form V_(c)sin ωt and -V_(c) sin ωt, where ω is the angular frequency of theoscillator output signal. The first carrier signal is supplied toterminal 74 of sensor 5, while the second carrier signal is supplied toterminal 76. Sensor output terminal 72 is connected to the non-invertinginput of a buffer amplifier 104. The output of the buffer amplifier isconnected to sensor terminal 80, the bootstrap diffusion contact.Through this connection, the parasitic capacitance is prevented fromloading the common node 72. A large resistance 106 (e.g., 3M) isconnected between a reference supply voltage VX and the non-invertinginput of buffer 104, to establish a d.c. operating point for the bridge.

The output of the buffer feeds a synchronous switching demodulator 110.The demodulator includes a switching circuit which is connected andresponsive to the output of the oscillator 100. The double-ended outputfrom the demodulator is converted to a single-ended output V_(o) by abuffer amplifier 120. The value of V_(o) is given by the formula V_(o)=V_(c) maG/k_(m) d₀, where V_(c) is the carrier amplitude, m is thebridge mass, a is acceleration, k_(m) is the beam's mechanical springconstant, d₀ is the nominal capacitor gap, and G is a scaling factorwhich accounts for buffer, demodulator and output amplifier gains.

Turning to FIG. 2E, a second embodiment is shown for signal conditioningcircuitry employing the sensor of FIGS. 2B and 2C. In contrast to theopen loop approach of FIG. 2D, the apparatus of FIG. 2E is aclosed-loop, force-balance accelerometer. To better illustrate theforce-balance principle, the bridge/differential-capacitor assembly ismodeled as a conductive mass 122 suspended between a first capacitorplate 124 and a second capacitor plate 126, which establishes first andsecond differential capacitances, the latter being shown as capacitors200 and 300, respectively. In the force-balance arrangement, capacitors200 and 300 serve two purposes. First, they provide the means wherebyelectrostatic balancing forces are applied to the mass 122, at theacceleration frequency. Secondly, they allow the displacement x of themass (i.e., the bridge mass) to be measured via the differentialcapacitance, at the carrier frequency. The negative feedback loopadjusts the output voltage V_(o) so that x=0 and inertial force appliedto the bridge equals the net electrostatic force which is applied. Theforce balance equation is as follows: ##EQU1## where m is the mass ofthe bridge,

ε₀ is the dielectric constant of air,

A is the capacitor plate area (each capacitor, nominal),

d₀ is the nominal, at rest, capacitor plate separation,

x is the change in the capacitor plate separation (i.e., the distancethe bridge moves due to the applied force),

V_(R) is the reference or d.c. offset voltage applied to the movableplates, and

V_(o) is the output voltage.

For x<<d₀, at a large loop gain, the output voltage V_(o), due toacceleration, is as follows: ##EQU2##

The output voltage is not sensitive to the spring constant of thestructure, k_(m), since the bridge remains undeflected. Full scaleadjustment to compensate for values of m, d₀ and A which vary fromnominal due to process variations can be made by trimming resistors R1and R2 in FIG. 2F.

The beam geometry is designed to minimize the mechanical springconstant, k_(m), so that a beam initially fabricated off-center will becentered automatically by a small percentage of the full scaleelectrostatic force. Then, a desired zero "g" output voltage level canbe established by trimming the relative carrier amplitudes. Taken to thelimit, the mass may be considered floating and self-centering; themechanical spring constant does, however, prevent the mass fromresponding to the carrier signal.

A more detailed design for the force-balance accelerometer of FIG. 2E isshown in FIG. 2F. The oscillator 100, carrier generator 102, buffer 104,and demodulator 110 are the same as the corresponding elements of FIG.7. The carrier generator is, however, a.c.-coupled to the sensor throughcapacitors 132 and 134. Capacitors 132 and 134 may typically be about30-50 pF each, to exhibit low impedance at the 1 MHZ carrier frequency.To establish a net electrostatic force on the sensor capacitor plates,input terminals 74 and 76 are connected, respectively, to positive andnegative offset (i.e., reference) supplies V_(R) and -V_(R), throughresistors 136 and 137, each typically being about 300k ohms. When thesensor capacitors 200 and 300 are equal (i.e., acceleration is zero),the electrostatic potential across the capacitors is balanced and equal.By contrast, acceleration causes the capacitors to have different valuesand the electrostatic potential on them to be unequal, causing a netunbalancing force. The demodulator detects this imbalance, which causesa change in the signal at the non-inverting input of amplifier 104, andsupplies a feedback signal through resistor 106, to create a netelectrostatic force to equalize the inertial force. Thus the feedbacksignal providing for force-balancing is supplied by connecting thejunction of resistor 106 and the second demodulator signal input at thebase of transistor 108 to the output of output buffer 120, instead of tofixed source VX.

Optionally, the circuit of FIG. 2F also has a resistor 140 and switch142 connected in series between node 74 and ground. Closure of theswitch will unbalance the input signals applied to nodes 74 and 76,which will apply a momentary electrostatic force on the bridge andproduce a corresponding shift in the output to re-center the bridge. Theoutput will be different if one or both capacitors fails (i.e., thebridge is untethered--broken--or the bridge is stuck). Thus, the closureof the switch can be used to test the proper operation of both thesensor and the circuitry.

Using the fabrication method of the present invention, the sensor ofFIG. 2A and the circuitry of FIG. 2B can be embodied in a single chip,thus reducing the size and cost of the accelerometer.

The fabrication method of the present invention combines processes forfabricating BIMOS circuitry and for fabricating suspendedmicrostructures in a mutually compatible manner. The overall methodcomprises approximately 330 individual steps.

Many of the approximately 330 individual steps are parts of processeswhich are well known in the art. For instance, the process of forming aphotoresist mask by means of photolithography is well known in the priorart and comprises six individual steps. The individual steps of thepresent invention can be considered to comprise 67 processes. Some ofthe 67 processes are standard processes which are known in the art. Thepresent invention lies in a novel combination of processes so as tofabricate a monolithic sensor as well as in the fact that some of theprocesses are novel in and of themselves.

On an even broader level, the overall method can be considered tocomprise 20 tasks, each task comprising one or more of the processes.The detailed discussion of the invention herein is divided into 20 taskheadings. The section under each task heading is further divided intoone or more processes. Where necessary, the individual steps of aprocess are discussed. However, as previously noted, many of theindividual steps are well known in the prior art and thus are notdescribed in great detail. Further, not all steps or processes areillustrated in distinct FIGS. in order to avoid obfuscation of theinvention.

Starting Material

The starting material for the method of the present invention is ap-doped silicon substrate having a thin p-doped epitaxial layerthereover of approximately 30 microns in thickness.

Task 1: N-Well Implant

This task involves implanting into the substrate the n-wells withinwhich the transistors will be formed. It comprises processes 1-5.

Process 1: Oxide 1

A thermal oxide is formed on the surface of the chip by conventionalthermal means. In particular, the chip is heated to approximately 1000°C. in an oxygen ambient environment, causing the silicon on the surfaceof the chip to oxidize, forming a thermal oxide, SiO₂, layer. The depthof oxidation is controlled by the temperature and exposure time. Thislayer is formed to a depth of approximately 3600 angstroms.

Process 2: N-Well Mask

This process involves patterning the oxide layer formed in step 1 intothe desired pattern of n-wells on the substrate. This process comprisesstandard photolithography steps followed by etching of the oxide layerin a buffered oxide etch (hydrofluoric acid) bath.

In particular, photolithography comprises the steps of (1) coating thechip with a blanket layer of organic photoresist, (2) placing a mask inthe form of the desired n-well pattern over, but not in contact with,the photoresist, (3) accurately aligning the mask over the chip,preferably in a stepper, so that all layers are formed in properposition relative to all other layers, (4) shining a light of specifiedwavelength through the mask onto the photoresist, causing the portionsof the photoresist exposed under the mask to be developed while theportions occluded by the mask are not developed, (5) removing the mask,and (6) spraying the chip with a chemical wash which washes away thedeveloped portion of the oxide layer.

After the six photolithography steps, the underlying oxide layer isexposed beneath the developed away portion of the photoresist. The chipis then dipped in a buffered oxide etch bath of hydrofluoric acid whichetches through the exposed portion of the oxide layer but does notaffect the portion of the oxide layer which is still covered byphotoresist.

Commonly, the photoresist layer would be removed immediately after theetching of the underlying layer. In the present process, however, thephotoresist is not yet removed for reasons which will be explainedshortly.

Process 3: N-Well Implant

In this process, phosphorus is blanket deposited on the chip to aconcentration of 5.0E12/cm². The phosphorus is implanted into thesubstrate only in the areas defined by the thermal oxide mask. Thephosphorus is deposited in a standard ion implantation process in whichthe chip is bombarded with a high energy beam of phosphorus particles(n-type particles). In the preferred embodiment, the particles areaccelerated to approximately 100 kiloelectron volts (KeV) of energy.

The photoresist mask which was formed in process 1, but not yet removed,serves as an additional protective mask in this n-well implant process.After the phosphorus is deposited, the photoresist can be stripped away.This can be done by either dipping the chip in a sulfuric acid etch bathor by dry plasma stripping. In the preferred embodiment of theinvention, it is done in an acid etch.

Process 4: N-Well Drive

At the completion of process 3, the phosphorus implanted in process 3 isdiffused into the substrate only to a very shallow depth. The phosphorusnow must be diffused (or driven) into the substrate to the desireddepth. The phosphorous can be diffused deeper into the substrate in athermal process. In the preferred embodiment of the invention, the chipis exposed to approximately 1250° C. for approximately 7 hours in anambient environment including a small amount of oxygen (to allow forsome oxidation) as well as a gas. The depth of diffusion can becontrolled by both the duration and temperature of the process.

Process 5: Oxide Strip

The n-wells having been implanted and driven to the desired depth, theprotective oxide layer formed in process 1 can now be removed. The chipis dipped into a buffered oxide etch bath. Since the photoresist maskhas been removed, all remaining oxide is etched away leaving the baresilicon with n-wells.

Task 2: Thick Oxide

This task relates to field threshold adjust in order to improve surfaceisolation between the components which will be formed in the chip. Itcomprises processes 6-14. Thick oxide will be formed around essentiallyall individual transistors to increase surface isolation. However, thedopant level must be increased beneath the thick oxide regions to reducesurface leakage. Accordingly, arsenic will be implanted in the regionsof the n-wells which will be covered by thick oxide (hereinafter n-fieldregions) to increase n-doping. In the other areas of the chip, boronwill be implanted under the regions where the thick oxide will be formed(hereinafter p-field regions) in order to increase p-doping in thoseareas.

Process 6: Pad Oxide

In this process, a thin layer of thermal oxide (approximately 500angstroms) is formed by oxidizing the surface of the chip as previouslydescribed with respect to process 1. This layer of oxide is formed inorder to protect the silicon from the nitride which will be deposited inprocess 7. Nitride will damage bare silicon.

Process 7: LPCVD Nitride Deposit

In this process, a layer of approximately 1200 angstroms of siliconnitride is deposited in a standard low pressure chemical vapordeposition process. In low pressure chemical vapor deposition, the chipis placed in a low temperature furnace containing an ambient gas. In thecase of a nitride deposition step, the ambient would include NH₃ andSiH₂ Cl₂. The vaporized particles deposit themselves onto any availablesurface such as the substrate surface.

Process 8: Thick Oxide Mask

In this process, a photoresist mask is formed using conventionalphotolithography as previously described with respect to process 2. Themask is patterned to define the field inactive regions (i.e., n-fieldand p-field regions) of the chip where a thick layer of oxide is to begrown in an upcoming process for increasing surface isolation. Once thephotoresist mask is formed, the nitride layer is etched by aconventional plasma etch. The photoresist mask is removed and the padoxide is then etched in a conventional buffered oxide etch. Only theportions exposed through the nitride layer are etched away.

Process 9: P-Field Mask

Since p-fields and n-fields must be doped with different impurities, aconventional photoresist mask is formed defining only those areasexposed through the thick oxide mask in which p-field regions are to beformed.

Process 10: P-Field Implant

In this process, boron particles (p-type particles) are implanted in thep-field regions exposed under the mask of photoresist to a concentrationof 5.5E13/cm² at 50 KeV by conventional ion implantation such as wasdescribed with respect to process 3. After the boron is implanted, thephotoresist mask is stripped away.

Process 11: N-Field Mask

This process is similar to process 9 except the mask defines the n-fieldregions rather than the p-field regions of the field area.

Process 12: N-Field Implant

In this process, arsenic is deposited to a concentration of 4.0E11/cm²by an ion implantation process to form the desired n-field regions inthe n-wells. In at least one preferred embodiment, the particles areaccelerated to 100 KeV. The photoresist is then stripped away.

FIG. 3 illustrates the state of the circuit region of the chip after thecompletion of the n-field implant of process 12, but before thephotoresist mask is removed. The p-doped substrate as well as the p-epilayer are shown collectively as 30. An exemplary n-well formed in TASK 1is shown at 40. The pad oxide layer (formed in process 6 and patternedin process 8) and the nitride layer (formed in process 7 and patternedin process 8) are shown collectively at 13. The n-field photoresist maskformed in process 11 is shown at 11 wherein openings 11a define then-field regions. The actual p-fields and n-fields, however, are notrepresented in this FIG. since they have not been fully formed yet. Inparticular, they have not been diffused to the desired depth yet.

Process 13: Thick Oxide

In this process, a blanket thick oxide layer of approximately 14,900angstroms is formed by thermal oxidation such as was described withrespect to process 1. The oxide formed in this process will only formwhere the bare silicon is exposed under the etched nitride layer formedin processes 7 and patterned in 8. Since the nitride formed in process 7and patterned in process 8 exposed the inactive regions of thesubstrate, the thick oxide forms only in those regions. The thick oxideimproves the electrical isolation of the transistors which will beformed on the chip from each other and from the aluminum leads whichwill be formed. The thick oxide layer is a field threshold adjust layer.

This thermal process also serves to diffuse the boron and arsenicimplanted in processes 10 and 12, respectively, thus completing theformation of the p-field and n-field regions.

Process 14: Nitride Strip

In this process, the chip is dipped in a phosphoric acid bath to removeall remaining nitride deposited in process 7. The chip is then dipped ina buffered oxide etch bath to remove the remaining oxide which wasformed in process 6. The buffer oxide strip, of course, will also stripaway some of the thick oxide layer formed in process 13. However, thatlayer is so thick that the vast majority of that layer remains after theoxide strip.

Task 3: Form Bases of Bipolar Transistors and Partially Form Sources andDrains of PMOS Transistors

This task comprises processes 15-20. In this task, the bases of thebipolar transistors are formed. Since the sources and drains of the PMOStransistors have similar doping requirements, they are also partiallyformed in this task. The more shallow source and drain diffusionsnecessary to completely form the source and drains of the PMOStransistors are formed later during TASK 7.

Process 15: Sacrificial Oxide

In this process, a 850 angstrom thick layer of sacrificial oxide isgrown on the chip by thermal oxidation. This sacrificial layer willserve to prevent channeling in the device as well as prevent damage fromoccurring in the upcoming base implant.

Process 16: Base Mask

In this process, a mask of photoresist is formed on top of thesacrificial oxide layer to define the desired bipolar transistor basesand PMOS sources and drains by standard photolithography steps aspreviously described. The sacrificial oxide layer is then etched intothe transistor base pattern in a buffered oxide etch bath also aspreviously described.

Process 17: Base Implant

In this process, p-type particles (boron particles) are ion implantedinto the substrate in the pattern dictated by the mask formed in process16. After the implantation, the photoresist is stripped away.

Process 18: Plate Mask

In this process, another photoresist mask is formed by standardphotolithography. The mask is formed so as to protect the circuitry areabut expose the moat area of the chip.

Process 19: Plate Implant

In this process, boron is ion implanted in the sensor area through themask formed in process 18. This implantation creates a more heavilyp-doped area (or plate) in the moat region. The plate serves as a fieldthreshold adjust to improve isolation between the arms of the sensor.The photoresist is stripped away after the plate implantation.

Process 20: Base Drive

In this process, the boron implanted in process 17 to form the bases ofthe bipolar transistors and to partially form the sources and drains ofthe PMOS transistors is diffused into the n-wells to the desired depthin a high-temperature diffusion process such as previously describedwith respect to process 4. The boron implanted in step 19 is alsodiffused at this time.

FIG. 4 illustrates the state of the chip and particularly the moatregion, after the completion of process 20. As before, the p-dopedsubstrate as well as the p-epi layer are shown collectively as 30. Theplate layer formed in process 19 is shown at 32. Dotted vertical line 34is the dividing line between the sensor area (or moat area) and thecircuit area of the chip. As can be seen in that figure, the plateappears only in the moat area. The sacrificial oxide layer formed inprocess 15 is shown at 36 and covers both the circuit area and thesensor area. An exemplary thick oxide region formed in task 2 (processes7-14) is shown at 38. Thick oxide region 38 in FIG. 4 is part of a ringof thick oxide (formed in task 2) which completely surrounds the moatarea. Of course, thick oxide has been deposited in other regions aswell. An exemplary n-well region in the circuit region is illustrated at40. A p-field formed in step 10 beneath thick oxide region 38 is shownat 42.

Task 4: Form Conductors From Moat Area to Circuit Area and BipolarTransistor Emitters

This task comprises processes 21-23. In this task, the emitters for thebipolar transistors are implanted. Further, n+runners between componentsof the sensor and the circuitry are also formed. These conductors areessentially long emitter regions and, thus, can be formed during thesame processes as the transistor emitters.

Process 21: Emit Mask

A layer of photoresist is deposited and patterned to define the bipolartransistor emitters as well as the n+runners (conductors) from thesensor area to the circuit area. These runners will electrically connectthe polysilicon sensor to the BIMOS circuitry.

Process 22: Emit Implant

In this process, arsenic is ion implanted to a concentration of6.18E15/cm² at 150 KeV to form the emitters and n+runners.

Process 23: Emit Drive

In this process, the arsenic is diffused in to the desired depth andconcentration in a high temperature diffusion process.

FIG. 5 shows the circuit region of the chip at the completion of process23. Reference numeral 17 denotes the base regions of the bipolartransistors and partially formed source and drain regions of the PMOStransistors formed in processes 16, 17 and 20. Reference numeral 19denotes the emitter regions formed in processes 21, 22 and 23. Referencenumeral 38 denotes the thick oxide layer formed in processes 8 and 13.Reference numeral 23 denotes an exemplary n-field formed in processes11, 12 and 13. Finally, reference numeral 42 denotes an exemplaryp-field formed in processes 9, 10 and 13.

Task 5: Form Gate Oxide Regions

This task comprises processes 24-26. In this task, the dielectric of theMOS transistors is formed.

Process 24: PVT Implant

In this process, boron is implanted to a concentration of 4.5E11/cm² at50 KeV. This blanket layer of boron is a threshold adjust for the PMOStransistors.

In the embodiment of the invention for manufacturing the monolithicaccelerometer shown in FIGS. 1A, 1B, 2A and 2B and disclosed in U.S.patent application Ser. No. 07/560,080, the circuitry comprises no NMOStransistors. Accordingly, the boron can be deposited in this stepwithout benefit of a mask. Although the boron is blanket deposited onthe chip, it only significantly affects the channel regions since theother regions of the chip are already so heavily doped that thisrelatively small addition of boron will not significantly affect theother regions. The additional steps necessary to adapt this process to adevice also having NMOS transistors, however, would be obvious to aperson of ordinary skill in the relevant art.

Process 25: Sacrificial Oxide Strip

In this process, the sacrificial oxide layer which was formed in process15 is now stripped away in a buffered oxide etch bath.

Process 26: Gate Oxide

In this process, a thermal oxide is grown on the chip to a depth of 725angstroms. This layer will comprise the capacitive oxide beneath thegates of the MOS transistors.

FIG. 6 illustrates the moat area of the chip after the completion ofprocess 26. As shown in FIG. 6, at this point, the moat area has threelayers above substrate 30. They are plate layer 32, n+conductors 44 andgate oxide blanket layer 46.

Task 6: Form BIMOS Gates

This task comprises processes 27-29. In this task, the polysilicon gatesof the BIMOS transistors are formed.

Process 27: Gate Polysilicon Deposit

In this process, a blanket layer of polysilicon is deposited on thechip. Polysilicon deposition techniques are well known in the art. Forinstance, the polysilicon may be deposited in a chemical vapordeposition technique involving exposing the chip in a high temperaturefurnace with an ambient containing vaporized silicon hydride (SiH₄). Thesilicon hydride decomposes in the high temperature and deposits onto allavailable surfaces, i.e., the chip. In the preferred embodiment of thepresent invention, the polysilicon is deposited to a depth of 5500angstroms. This blanket of polysilicon will be formed into the gates ofthe MOS transistors in upcoming processes.

Process 28: POCl₃ Polysilicon Doping

In this process, the polysilicon deposited in process 27 is highly doped(20 ohms/sq) with POCl₃ in order to increase its conductivity. In thepreferred embodiment of the invention, the polysilicon is doped in adeposition cycle of a diffusion process in which the chip is placed in ahigh temperature furnace with ambient POCl₃. Although similar to achemical vapor deposition technique, this process is not considered achemical vapor deposition because, instead of depositing a new layer ofmaterial, this process dopes the prior existing layer with an impurity.

Process 29: Polysilicon Mask

In this process, a photoresist layer is formed over the polysilicon andpatterned into the desired gate pattern. The polysilicon is then etchedin a dry plasma etch to form the gate regions.

FIG. 7 illustrates the sensor area of the chip after completion ofprocess 29. In addition to forming the gate regions, an island of gatepolysilicon 48 is also formed over the moat area. This polysiliconisland will act as an etch stop when removing BPSG in a subsequentprocess. Near the end of the fabrication method, a process will removeall gate polysilicon from the sensor region except for a narrow ring.

Task 7: Form Shallow P-Type Source and Drain Regions for BIMOSTransistors

This task comprises processes 30 and 31, in which the source and drainregions of the PMOS transistors are formed.

Process 30: P-Type Source and Drain Mask

In this process, a photoresist mask is formed by standardphotolithography steps to define the source and drain regions of thePMOS transistors.

Process 31: PSD Implant

In this process, boron (p-type) is ion implanted to a concentration of1.5E13/cm² at 40 KeV. The photoresist is stripped subsequent to theboron implantation.

Task 8: BPSG Planarization

At this stage in the fabrication of the device, the surface topographyof the chip is relatively severe (i.e., rough). In subsequent processes,it will be necessary to form metal leads on the surface of the chip. Itis preferable that the metal be deposited on a relatively smoothsurface. Accordingly, in processes 32-35, a borophosphosilicate glass(BPSG), which essentially comprises SiO₂ with small amounts of boron andphosphorus, is deposited and reflowed to provide a smoother surface forthe metallization process. Otherwise, the severe topography of thesurface of the chip would make it difficult to deposit metal leadswithout cracking.

Process 32: LPCVD Nitride Deposition

In this process, a 200 angstrom thick layer of nitride is deposited onthe surface of the chip by low-pressure chemical vapor deposition. Thisnitride layer protects the underlying gate oxide from the BPSG layerwhich will be deposited in process 33. The nitride prevents the BPSGfrom diffusing into and beyond the gate oxide layer.

Process 33: BPSG Deposition

In this process, a 5500 angstrom thick layer of borophosphosilicateglass is plasma enhanced chemical vapor deposited. The BPSG layer willbe reflowed to provide a planarization layer for reducing the severityof the topography such that metal interconnects and contacts can bedeposited subsequently with reduced risk of cracking. The BPSG will bereflowed to planarize the chip surface in process 34.

Process 34: Source and Drain Drive

In this process, the boron deposited as source and drain regions isdiffused to the desired depth and concentration. In at least onepreferred embodiment, the chip is exposed to 1000° C. for approximately2 hours. This process is actually part of both task 8 (planarization)and task 7 (formation of the source and drain regions). This process isperformed subsequent to the BPSG deposition to accomplish source/draindrive as well as BPSG reflow (i.e., melt and resolidify the BPSG so itforms a smooth planar surface upon which the metal interconnects can beformed).

Process 35: LPCVD Nitride Deposition

In this process, another 200 angstroms of nitride are deposited over theBPSG layer. This nitride layer will act as an etch stop in the circuitarea when it becomes necessary to wet etch through dielectrics depositedfor the purposes of constructing the sensor. Dielectrics are commonlylaid down as a blanket and thus the sensor dielectrics will also beformed in the circuit area at some point. Accordingly, they must beremoved from the circuit area. This nitride layer will act as an etchstop when it becomes necessary to remove those dielectrics.

FIG. 8 illustrates the condition of the sensor area of the chip afterthe completion of process 35. The nitride layer deposited in process 32is shown at 50, the BPSG layer formed in process 33 is shown at 52, andthe nitride layer formed in process 35 is shown at 54.

Task 9: Clear Moat Area

As can be seen from FIG. 8, at this stage in the fabrication method ofthe present invention the moat region is covered with a number of layersof dielectric which were deposited for BIMOS circuitry formation. Inprocesses 36 through 40, the unnecessary dielectrics will be removedfrom the moat area and the surface concentration of the p-regions willbe increased to reduce the probability of surface leakage between then+runners. Further, after the moat area is cleared of all BIMOSdielectrics and its dopant concentration has been increased, severaldielectric layers for the moat are formed.

Process 36: Moat Mask

In this process, a photoresist layer is deposited and patterned toexpose only the moat region. Then, the nitride layer deposited inprocess 35 is removed in a dry etch process, the BPSG planarizationlayer deposited in process 33 is removed in a wet etch process, thenitride layer formed in process 32 is removed in another dry etchprocess and, finally, the polysilicon layer deposited in process 27 isremoved in a dry etch process. At this point, the gate oxide is nowexposed in the moat region.

Process 37: Moat Implant

In this process, more boron is ion implanted in the moat area (theremainder of the chip is still protected by the photoresist). The boronis implanted to a concentration of 5E12/cm² at 50 KeV. This boronimplantation procedure serves the same function as the plate implantprocedure of process 19, i.e., as a field threshold adjust to increasecomponentry electrical isolation. The process is split into two separateimplantations because, at this point, the initial plate implant has beensubstantially depleted from the surface due to exposure to severalthermal operations and the segregation nature of boron.

FIG. 9 shows the chip after the completion process 37. As can be seen,the moat area is exposed down to the gate oxide 48.

Process 38: LTO Deposition

In this process, a 2,000 angstrom thick layer of low temperature oxideis deposited over the chip. This increases the total oxide thicknessfrom about 600 angstroms to about 2600 angstroms in the field regionsand ensures adequate surface passivation. In this process, the oxide isnot formed by a thermal process as previously described wherein theexposed silicon surfaces oxidize. Instead, in this process, the oxide isdeposited in a chemical vapor deposition type process. Essentially, thechip is placed in a low temperature furnace with a silicon compound andoxygen ambient. The ambient oxide precipitates onto the surface of thesilicon. This process does not use up any silicon on the substrate.

Process 39: Densification

The low temperature oxide deposited in process 38 is now densified toslow down its etching. Essentially, the chip is placed in a hightemperature furnace for a specified period to densify the oxide.

Process 40: LPCVD Nitride Deposition

A 1200 angstrom thick layer of nitride is deposited over the densifiedlow temperature oxide. This layer will act as an etch stop with respectto the etching of another low temperature oxide layer which will bedeposited over this nitride layer in process 41. This nitride layer willalso permit the nitride-to-nitride sealing discussed in Task 19.

FIG. 10 shows the chip after the completion of process 40. As showntherein, a layer of low temperature oxide 56 has been deposited followedby a layer of nitride 58.

Task 10: Form Spacer Oxide

The microstructure should be able to withstand normal operatingconditions after the complete chip has been fabricated and packaged. Forinstance, the monolithic accelerometer which is contemplated forfabrication by the method of the present invention is expected towithstand accelerative forces on the order of 100 g. Nevertheless,during fabrication, the microstructure undergoes processes and isexposed to environmental conditions which are significantly more harshthan it is likely to encounter during operation. Thus, precautionarymeasures must be built into the microstructure as well as the method forfabricating the microstructure in order to reduce the possibility ofdamaging the microstructure during fabrication.

The spacer oxide serves two functions. First, the polysiliconmicrostructure will be deposited and formed over the spacer oxide andthen the spacer oxide will be etched away from underneath themicrostructure leaving it suspended. The spacer oxide is not removeduntil essentially the very end of the method. Accordingly, a secondfunction of the spacer oxide is to rigidly support the suspendedmicrostructure which otherwise might be damaged when exposed to theconditions and processes of fabrication.

The formation of the spacer oxide comprises process 41-43.

Process 41: Spacer Oxide Deposition

In this process, the spacer low temperature oxide (LTO) which willsupport the microstructure is deposited by chemical vapor deposition.This layer is grown to approximately 16,000 angstroms.

Process 42: Densification

In this process, the chip is exposed to a high temperature for anextended period of time in order to densify the spacer LTO layer to slowdown its etching. This will allow for much more accurate etching of thespacer oxide.

Process 43: Bumps Mask

In this process, a photoresist layer is deposited over the spacer LTOand patterned to form small openings interspersed in the sensor area.The spacer oxide is then etched in a buffer oxide etch bath. The chip isonly exposed to the buffer oxide etch for a limited period of time sothat only small divots will be taken out of the top surface of thespacer LTO rather than etching completely through the spacer LTO down tothe underlying nitride layer. Thus, when the microstructure is formedover the spacer LTO, there will be small bumps in its bottom surfaceadjacent to the position of the divots in the spacer oxide.

These bumps, will serve to minimize surface area contact of themicrostructure to the chip during and after removal of the spacer oxide.During and after the etch for removing the spacer oxide to leave themicrostructure suspended, the relatively delicate microstructure can bebent such that it comes in contact with the underlying substrate. Thisis undesirable since the microstructure has a tendency to stick to thesubstrate. By placing small bumps in the bottom surface of themicrostructure, if and when the bottom surface of the microstructurecomes in contact with the substrate, only the bumps will contact thesurface thereby minimizing the contact surface area and the likelihoodof sticking.

FIG. 11 shows the chip after the completion of process 43. As showntherein, a thick layer of spacer oxide 60 has been deposited over theentire chip and small divots such as divot 61 have been formed in itsupper surface.

Task 11: Anchor Formation

In the exemplary accelerometer illustrated in FIGS. 1A, 1B and 2A, themicrostructure is suspended from four anchors, and the fixed arms areindividually anchored to the substrate in the same manner. Processes 44and 45 relate to the preparation for forming the anchors of themicrostructure.

Process 44: Anchor Mask

In this process, a photoresist mask is formed defining the anchors fromwhich the polysilicon microstructure will be suspended. The spaceroxide, nitride, and additional underlying oxides are dry etched, thusexposing the n+runners beneath, such as runner 44 (see FIG. 12).

Process 45: Anchor Implant

In this process, phosphorous is implanted to a concentration of4.0E15/cm² in an ion implantation process at 30 KeV. The phosphorous isimplanted so as to allow the polysilicon anchors, when formed, to makegood ohmic contact to the underlying n+runner, e.g., 44. The phosphorousimplantation increases the phosphorous concentration at thepolysilicon/silicon interface, encouraging uniform recrystallization atthe anchor point and reducing anchor resistance.

FIG. 12 shows the chip after the completion of step 45, includingexemplary anchor opening 59.

Task 12: Polysilicon Microstructure Formation

Processes 46-49 relate to the formation of the suspended microstructure.However, as noted earlier, the microstructure will not be suspended, butwill be supported by the spacer LTO, until essentially the end of thefabrication method.

Process 46: Sensor Polysilicon Deposition

In this process, a 20,000 angstrom thick layer of polysilicon isdeposited over the spacer oxide in a low pressure chemical vapordeposition process. This is the polysilicon layer from which themicrostructure will be formed. A low deposition temperature is used toproduce a partially amorphous film.

Process 47: Sensor Polysilicon Implant

In this process, the microstructure polysilicon is made more conductiveby ion implanting phosphorous.

Process 48: Polysilicon Ramped Anneal

When the polysilicon is deposited, it is substantially amorphous. Inorder to form the polysilicon to the desired tensile stress, it isannealed. The chip is heated for an extended period in a nitrogenambient (N₂). The annealing step also drives the phosphorous dopant intothe polysilicon as in a standard implant drive process. In the preferredembodiment, the polysilicon is annealed to result in a tensile stress ofapproximately 6E8dynes/cm² in order to maintain a relatively stablespring constant for the finished microstructure. The resultingpolysilicon sheet resistance is approximately 90 to 160 ohms/sq.

FIG. 13 shows the chip after completion of step 48. As shown therein, a20,000 angstrom thick layer of polysilicon 62 has been deposited overthe chip. As shown in FIG. 13, an anchor 64 is formed in anchor well 59.

Process 49: Microstructure Mask

In this process, a photoresist mask is formed to remove all polysiliconfrom the circuit area and to form the desired microstructure shape inthe moat area. The polysilicon is then dry etched. FIG. 14 illustratesthe chip after the completion of process 49.

Task 13: Remove Spacer Oxide from Circuit Area

This task comprises only process 50.

Process 50: MOBE Mask

In this process, a photoresist layer is deposited and masked for etchingthe spacer oxide layer 60. The term "MOBE" is an arbitrary designationfor this particular mask. The term "MOBE" is a contraction of "moat andbeam" and was selected because this mask is formed so as to cover themicrostructure (or beam) but to allow the spacer oxide 60 to be removedfrom the circuit area. The mask, different than the moat mask used inprocess 36. After the photoresist is patterned, the chip is deposited inthe buffer oxide etch bath in which the spacer oxide is selectivelyetched away down to underlying nitride layer 58. The LPCVD nitride layer58 acts as an etch stop for the etching of the oxide.

FIG. 15 illustrates the chip subsequent to the completion of step 50.

Task 14: Deposit Oxide Layer to Protect Microstructure

This task comprises only a single process.

Process 51: Low Temperature Oxide Deposition

In this process, a 2,000 angstrom thick layer of low temperature oxideis deposited by chemical vapor deposition. This oxide layer will serveto protect the microstructure from an upcoming deposition of platinumfor forming the electrical contacts in the circuit area as well asbetween the sensor and the circuitry. Without the thin layer of LTO, theplatinum would react with the polysilicon during platinum silicidationand alter the polysilicon's mechanical and electrical properties.

FIG. 16 shows the 2,000 angstrom low temperature oxide layer at 66.

Task 15: Remove Sensor Dielectrics from Circuit Area

This task comprises only a single process.

Process 52: MOSIN Mask

A patterned photoresist layer is formed using standard photolithographysteps to define a large island in the moat area where all sensordielectrics will be preserved while the remainder of the photoresist isdeveloped away so as to allow the etching away of all sensor dielectricsfrom the circuit area of the chip. Two masks are used here, a MOSIN maskand the MOBE mask, to provide extra photoresist protection on the sharpvertical steps of the polysilicon sensor. The term "MOSIN" was chosen todistinguish this mask from the MOAT mask and the MOBE mask. The term"MOSIN" is a contraction of "moat and silicon nitride" since the maskwill be used to etch all the way down to nitride layer 54. In thisprocess, the MOBE mask is used first followed by the MOSIN mask. TheMOBE mask is slightly smaller than the MOSIN mask such that twophotoresist patterns are visible. The sensor dielectrics which areetched away from the circuit area in this process comprise the lowtemperature oxide layer deposited in process 51, the nitride layerdeposited in process 40 and the low temperature oxide layer deposited inprocess 38. The two oxide layers are removed by separate wet etchprocesses (buffer oxide etch). The nitride layer is removed in a dryplasma etch process. The nitride layer 54 underlying the three layerswhich are etched in this process that was deposited during process 35remains. This layer, 54, serves as an etch stop for the etching of oxidelayer 56. The three layers etched away in this process are removed inorder to enable laser trimming in the circuit area. The photoresist isthen removed after the three layers are etched.

FIG. 17 shows the chip after the completion of process 52. As showntherein, layers 66, 58 and 56 have been removed from the circuit area,thus exposing nitride layer 54.

Task 16: Contact Mask

This task comprises only process 53.

Process 53: Contact Mask

Like MOSIN mask in process 52, contact mask utilizes two separatephotoresist layers (contact and MOBE) to provide sufficient photoresistcoverage on the sharp vertical steps in the sensor area. First MOBE maskis applied to cover the moat area and leave the circuitry regionsexposed. Then, contact mask is applied. The contact mask is developed todefine openings to the transistors for metal contacts and openings tothe n+runners in the moat area for metal contacts and also to provideadditional photoresist coverage for the microstructure in the moat area.After the photoresist masks have been formed, four layers must be etchedthrough. They are nitride layer 54, BPSG layer 52, nitride layer 50 andgate oxide layer 46. All four layers are etched in a single long plasmaetch which exposes the substrate in the defined contact areas such asthe bare n+runner in opening 67 in FIG. 19.

FIG. 18 illustrates the circuit region of the chip after the completionof process 53. Reference numeral 48 indicates the gate polysilicon.Reference numeral 46 indicates the gate oxide. The shallow source anddrain diffusion regions of the PMOS transistors are shown at 33. TheBPSG layer is shown at 52. The remaining numerals refer to regionspreviously discussed with respect to at least FIG. 5 and have beendefined in the discussion of FIG. 5.

Task 17: Form Platinum Silicide in Contact Areas

This task comprises processes 54-56. The purpose of this task is to forma platinum silicide layer in the metal contact openings, e.g., opening67 in FIG. 19, so as to provide good ohmic contact with the electricalleads which will be formed in subsequent processes.

Process 54: Platinum Deposit

In this process, platinum is deposited on the chip by a standardhigh-vacuum sputtering process. The platinum is deposited to a depth of400 angstroms.

Process 55: Platinum Sinter

The platinum formed in process 54 is sintered in this process so as tocause the platinum to react with the silicon on the surface of thesubstrate to form platinum silicide. The process essentially comprisesexposing the chip to a high temperature. The platinum silicidizes onlywhere it contacts the bare substrate, e.g., in the contact openings.

Process 56: Platinum Strip

In this process, the platinum which has not been silicidized is removed.Specifically, the chip is placed in a nitric-HCL acid bath which stripsaway unsilicidized platinum leaving platinum silicide in the contactopenings.

FIG. 19 shows the chip after the completion of process 56 illustratingsilicidized contact opening 67.

Task 18: Metallization

The metallization task comprises processes 57-61. In this task, themetal leads are formed to connect the various contact openings to eachother so as to couple the transistors together in the desired circuitpattern. The thin film trimmable resistors also are formed.

Process 57: SiCr Sputter

In this process, a blanket of SiCr is formed by a standard sputterdeposition process. The SiCr will be used to form laser trimmableresistors. The SiCr is deposited to a sheet resistance of 1,000-1,200ohms/sq.

Process 58: Metal Sputter

In this process, titanium tungsten (TiW) and aluminum/copper (AlCu) aredeposited in two separate sputtering processes. The two metals will beformed into the electric leads in the following process.

Process 59: Metal Mask

In this process, a photoresist mask is formed defining the desired metalleads. Two consecutive exposures are required with two different masksto insure complete removal of the photoresist (and ultimately the metal)from the two micrometer deep sensor gaps. The first mask exposed is astandard metal mask to connect the BIMOS circuitry on the chip. Then, amoat mask is exposed in the same resist to facilitate removal of allresist from the sensor area during development. Only the sensor area isopened by moat mask so only the sensor area gets over exposed. The AlCuis then etched by a wet etch process in a bath of phosphoric aceticnitric (PAN) acid. This is followed by a wet etch of the TiW in hydrogenperoxide leaving the desired electrical leads. The photoresist is thenstripped away.

Process 60: Thin Film Mask

Photoresist is deposited and patterned to define the desired resistors.The SiCr layer is dry etched in accordance with the photoresist mask toform the resistors. The photoresist is then stripped away.

Thin film mask, like the metal mask in process 59, uses a secondexposure with moat mask to facilitate clearing the sensor area ofphotoresist and, therefore, thin film material also.

Process 61: Alloy

In this process, the chip is exposed in a high temperature furnace tocause the TiW and AlCu to react with the platinum silicide in thecontact openings so as to form good ohmic contact between the substrateand the electrical leads.

FIG. 20 illustrates the circuit after the completion of process 61. Asshown therein, metal leads, such as lead 68, have been formed in contactopenings such as opening 67. The leads extend over nitride layer 54 toconnect the contact openings, thus coupling the transistors into thedesired circuit.

Task 19: Passivation

This task comprises processes 62-66. The passivation used in the methodof this invention is deposited in two separate steps to allow anitride-to-nitride seal to be formed around the sensor to protect thecircuitry from the very long etching process which will follow tocompletely undercut the spacer LTO and release the sensor. Thepassivation layers will serve to protect the metal from scratching andalso to protect the circuitry from moisture, ionic contamination, etc.The passivation, however, cannot exist in the bond pad openings becausemetal must be bonded to the bond pads. The passivation also must beremoved from the microstructure so as not to affect its free movementunder accelerative forces.

Process 62: Plasma Oxide Deposit

In this process, plasma oxide is deposited to a thickness of 5,000angstroms by plasma enhanced chemical vapor deposition. A small amountof phosphorous is contained in the oxygen plasma so as to form part ofthe oxide layer.

Process 63: Passivation Mask

In this process, the oxide layer formed in process 62 is patterned toset up the nitride-to-nitride seal around the perimeter of the sensorand also to open up the circuit area bond pads. Accordingly, aphotoresist mask is formed and patterned to define (1) a channel aroundthe perimeter of the sensor and (2) the bond pads. The oxide layer isetched in a buffer oxide etch bath and the photoresist is then strippedaway.

FIG. 21 illustrates the chip subsequent to the completion of process 63.As shown therein, a plasma oxide layer 70 has been formed over the chipand layer 70 has been etched to define openings such as sensor perimeteropening 72 and bond pad opening 74.

Process 64: Plasma Nitride Deposit

At this point, another layer of nitride 76 is deposited to a thicknessof approximately 5,000 angstroms by plasma enhanced chemical vapordeposition. This sets up a nitride-to-nitride seal around the sensorarea. The nitride-to-nitride seal protects the circuit area from thefinal long wet etch used to remove the spacer LTO from under thepolysilicon sensor. The seal is formed between this nitride layer 76(see FIG. 22) and the LPCVD nitride layer 58 that was deposited inprocess 40.

Process 65: Back Etch

This is a standard process and is not actually related to thenitride-to-nitride seal task 19. However, it is performed at this point,i.e., before the nitride layer deposited in process 64 is removed,because the nitride layer adds extra protection during the back etch. Inany event, the back etch involves blanket coating the front of the chipwith photoresist and etching all dielectrics from the back of the chipin a long series of wet and dry etches.

Process 66: NTPAS Mask

It is now necessary to remove the nitride deposited in process 64 fromthe sensor area as well as from the bond pads, e.g., bond pad 74. Aphotoresist mask is formed to open the bond pads and the sensor. Thenitride layer 76 is etched in a dry etching process. As noted above,this etching process is performed after the back etch because thenitride layer provides added protection during the back etch.

FIG. 22 illustrates the chip after the completion of process 66. Asshown therein, a nitride layer 76 has been formed over the chip and hasbeen etched to open up the metal leads in the bond pad regions such aslead 68 and to open up the sensor.

Task 20: Release Microstructure

This final task comprises only process 67 in which the spacer oxidelayer is removed, thus releasing the microstructure into its finalsuspended condition as illustrated in FIGS. 1A, 1B and 2A.

Process 67: Microstructure Release Mask

In this process, a photoresist mask is formed to entirely cover thecircuitry area and most of the moat area with a few holes in the moatarea adjacent to sections where the spacer oxide is exposed, i.e.,sections where the polysilicon has been removed to form themicrostructure shape. The holes in the photoresist are placed such thatthey are directly adjacent to the edges of the overlying microstructure.

The chip is then placed in a buffered etch oxide bath such that theoxide layer is etched where it is exposed under the photoresist mask.The chip is left in the bath for an extended period such that the oxideis etched slightly beyond the dimensions of the hole in the photoresistand extend a few microns beneath the edges of the microstructure.

The photoresist is then removed and another layer of photoresist isdeposited on the chip. This layer of photoresist fills in the holes nowformed in the oxide layer as well as fills in the voids between portionsof the etched polysilicon microstructure. The photoresist is thenexposed without a mask to develop away most of the photoresist. However,the portions of the photoresist which have filled in the edges of theholes and which extend under the edges of the microstructure are notdeveloped away because these edges are occluded by the polysiliconmicrostructure. If desired, a mask can be used so as also to leave somephotoresist bridges in polysilicon layer 62 in the gaps betweennon-contacting portions of the polysilicon microstructure, e.g., betweenmobile arms 14.

The remaining spacer oxide 60 is then removed in a buffer oxide etch.This bath also removes oxide layers 66 and 70 which were earlier left inthe moat area coating the microstructure. The buffered oxide bath doesnot affect the polysilicon or the photoresist. Accordingly, thephotoresist bridges in the gaps between non-contacting portions of themicrostructure as well as the photoresist pedestals formed underneaththe microstructure still remain after the spacer oxide is removed. Thephotoresist pedestals which formed at the edges of the holes beneath themicrostructure vertically support the microstructure preventing it frombending and contacting the underlying substrate. The photoresist bridgesleft in the gaps between non-contacting portions of the microstructureprovide lateral support preventing such portions from laterally bendingand contacting each other.

Without the photoresist pedestals and bridges, portions of themicrostructure would be extremely prone to bending and contacting thesubstrate and/or other portions of the microstructure during the dryingprocedure after the buffer oxide wet etch due to liquid surface tensioneffects. The photoresist pedestals and bridges significantly reduce thepossibility of portions of the microstructure contacting the substrateor other portions of the microstructure. Further, even if contactoccurs, the bumps formed on the bottom surface of the microstructureduring process 43 significantly reduce surface area contact thussignificantly reducing the possibility of sticking. Accordingly, thephotoresist pedestals and bridges as well as the bumps significantlyincrease the device yield of the method of the present invention.

The photoresist pedestals and bridges are removed in a long oxygenplasma stripping process which does not present any liquid surfacetension problems.

Process 67 of the present invention is more fully discussed in U.S.patent application Ser. No. 07/872,037 referenced above. Accordingly, itwill only be briefly discussed here. For a more complete understandingof this process and its many steps, reference should be made to theaforementioned patent application.

At this point, the microstructure is now suspended in its final form asillustrated in FIG. 20.

It should be understood that the method of the present invention can bereadily adapted for circuitry having NMOS transistors in addition to thePMOS and bipolar transistors discussed in the preferred embodimentdisclosed herein. It should further be understood that the method of thepresent invention can be used to fabricate many kinds of chips embodyinga microstructure as well as circuitry and is not limited to theaccelerometer discussed herein.

Having thus described a few particular embodiments of the invention,various alterations, modifications and improvements will readily occurto those skilled in the art. Such alterations, modifications andimprovements as are made obvious by this disclosure are intended to bepart of this description though not expressly stated herein, and areintended to be within the spirit and scope of the invention.Accordingly, the foregoing description is by way of example only, andnot limiting. The invention is limited only as defined in the followingclaims and equivalents thereto.

What is claimed is:
 1. A process for fabricating a monolithic device,comprising the steps of:(1) providing a substrate having a first surfaceregion for formation of a circuit and a second surface region forformation of a suspended microstructure; (2) forming transistors in thefirst surface region of said substrate; (3) forming a spacer layer overthe second surface region of said substrate; (4) forming at least oneanchor opening in said spacer layer; (5) forming a patterned polysiliconlayer over said spacer layer and said anchor opening to define saidmicrostructure; (6) forming conductive paths for electricallyinterconnecting said transistors to form said circuit and forelectrically interconnecting said microstructure to said circuit; and(7) removing said spacer layer to release said suspended microstructure.2. A process for fabricating a monolithic device as defined in claim 1wherein the step of forming transistors includes forming an oxide layerand other dielectric layers over the first surface region and the secondsurface region of said substrate and wherein the process furtherincludes the steps of:forming an etch stop layer over the first surfaceregion of said substrate following step (2), removing said otherdielectric layers from the second surface region of said substrate, andforming a passivating layer over the second surface region of saidsubstrate.
 3. A process for fabricating a monolithic device as definedin claim 1 further including the step of forming a protective layer oversaid polysilicon layer in the second surface region of said substrateprior to the step of forming conductive paths.
 4. A process forfabricating a monolithic device as defined in claim 3 wherein saidprotective layer comprises silicon oxide.
 5. A process for fabricating amonolithic device as defined in claim 1 further including the step offorming a passivating layer over the first surface region of saidsubstrate before the step of removing said spacer layer.
 6. A processfor fabricating a monolithic device as defined in claim 1 wherein thestep of forming a spacer layer includes forming a silicon oxide spacerlayer.
 7. A process for fabricating a monolithic device as defined inclaim 1 wherein the step of forming a patterned polysilicon layerincludes the steps of depositing a polysilicon layer over said spacerlayer and etching said polysilicon layer to form said patternedpolysilicon layer.
 8. A process for fabricating a monolithic device asdefined in claim 1 wherein the step of forming a patterned polysiliconlayer includes implanting ions into said polysilicon layer to establisha desired conductivity of said polysilicon layer.
 9. A method forfabricating a monolithic device as defined in claim 1 further includingthe step of removing said spacer layer from the first surface region ofsaid substrate.
 10. A process for fabricating a monolithic device,comprising the steps of:(1) providing a substrate having a first surfaceregion for formation of a circuit and a second surface region forformation of a suspended microstructure; (2) forming transistors in thefirst surface region of said substrate, the step of forming transistorsincluding forming an oxide layer and other dielectric layers over thefirst surface region and the second surface region of said substrate;(3) forming an etch stop layer over the first surface region of saidsubstrate; (4) removing said other dielectric layers from the secondsurface region of said substrate; (5) forming a passivating layer overthe second surface region of said substrate; (6) forming a spacer layerover the second surface region of said substrate; (7) forming at leastone anchor opening in said spacer layer; (8) forming a patternedpolysilicon layer over said spacer layer in the second surface region ofsaid substrate, said patterned polysilicon layer defining saidmicrostructure; (9) forming a protective layer over said patternedpolysilicon layer in the second surface region of said substrate; (10)forming final conductive paths for electrically interconnecting saidtransistors to form said circuit and for electrically interconnectingsaid microstructure to said circuit; and (11) removing said spacer layerto release said suspended microstructure.
 11. A process for fabricatinga monolithic device as defined in claim 10 wherein the step of forming aspacer layer includes forming a silicon oxide spacer layer.
 12. Aprocess for fabricating a monolithic device as defined in claim 10wherein the step of forming a patterned polysilicon layer includesdepositing a polysilicon layer over said spacer layer and etching saidpolysilicon layer to form said patterned polysilicon layer.
 13. Aprocess for fabricating a monolithic device as defined in claim 12wherein the step of forming a patterned polysilicon layer furtherincludes implanting ions into said polysilicon layer to established adesired conductivity of said polysilicon layer.
 14. A process forfabricating a monolithic device as defined in claim 10 further includinga step of removing said spacer layer from the first surface region ofsaid substrate.
 15. A process for fabricating a monolithic device asdefined in claim 10 further including forming a passivating layer overthe first surface region of said substrate prior to removing said spacerlayer.
 16. A process for fabricating a suspended microstructure on asubstrate, comprising the steps of:(1) forming a spacer layer over thesubstrate; (2) forming at least one anchor opening in said spacer layer;(3) forming a polysilicon layer over said spacer layer and said anchoropening; (4) implanting ions into said polysilicon layer to establish adesired conductivity of said polysilicon layer; (5) patterning saidpolysilicon layer to define the suspended microstructure; and (6)removing said spacer layer to release said suspended microstructure.